Corrosion-resistant high-strength low-alloy steels

ABSTRACT

Low-alloy steels having five to eight times the atmospheric corrosion resistance of carbon steels, 80,000 psi minimum yield strengths and an overall desirable balance of mechanical properties produced directly off the hot-mill have the following chemistry: carbon, .04 percent to .10 percent; manganese, .40 percent to 1.80 percent; sulfur, .03 percent maximum; aluminum, .015 percent minimum; chromium, .90 percent to 1.20 percent copper, .30 percent to .50 percent; silicon, .50 percent to 1.20 percent; phophorus, .10 percent to .15 percent; columbium, .015 percent to .040 percent; and zirconium, .04 percent to .12 percent or a total content of rare earth elements, such as cerium and lanthanum, such that the weight ratio of total rare earths to sulfur is at least 2.8 to 1. In processing the steel to strip or plate, it is hot-rolled so as to have a finishing temperature between its A3 temperature and 1,700* F, cooled at a rate between 20* F/sec. and 45* F/sec. and coiled or piled at a temperature of 1,100* F + OR - 100* F.

orchynslcy et al.

[ll] [45] Jan. 22 19M [75] Inventors: Michael Korchynsky, Bethel Park;

John R Bell; Richard J. Cover, both of Pittsburgh, all of Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa.

22 Filed: on. s, 1972 [21] App1.No.: 296,721

Related US. Application Data [62] Division of Ser. No. 123,470, March 11, 1971, Pat.

Primary ExaminerW. W. Stallard Attorney, Agent, or Firm-G. R. Harris; T. A. Zalenski [57] ABSTRACT Low-alloy steels having five to eight times the atmospheric corrosion resistance of carbon steels, 80,000 psi minimum yield strengths and an overall desirable balance of mechanical properties produced directly off the hot-mill have the following chemistry: carbon, .04 percent to .10 percent; manganese, .40 percent to 1.80 percent; sulfur, .03 percent maximum; aluminum, .015 percent minimum; chromium, .90 percent to 1.20 percent copper, .30 percent to .50 percent; silicon, .50 percent to 1.20 percent; phophorus, .10 percent to .15 percent; columbium, .015 percent to .040 percent; and zirconium, .04 percent to .12 percent or a total content of rare earth elements, such as cerium and lanthanum, such that the weight ratio of total rare earths to sulfur is at least 2.8 to 1. In processing the steel to strip or plate, it is hot-rolled so as to have a finishing temperature between its A temperature and 1,700 F, cooled at a rate between 20 F/sec. and 45 F/sec. and coiled or piled at a temperature of 1,l00 F 100 F.

4 Claims, No Drawings CORROSION-RESISTANT HIGH-STRENGTH LOW-ALLOY STEELS This is a divison of application Ser. No. 123,470 filed Mar. 11, 1971, now U.S. Pat. No. 3,711,340.

' The present invention relates to high-strength lowalloy steels resistant to atmospheric corrosion.

A class of high-strength low-alloy steels known as weathering steels has been commerically available for the last few years. These steels typically possess yield strengths between 50,000 psi and 60,000 psi and exhibit resistance to atmospheric corrosion four to eight times as great as carbon steel. The weathering steels generally are alloyed with small amounts of several of the following elements: copper, chromium, phosphorous, silicon and nickel. They achieve their corrosion resistance by-forming, upon prolonged exposure, a tight non-flaking oxide coating and are used in bridges, buildings, sign and lamp posts, transmission towers, railraod cars, highway rails, architectural curtain walls, etc.

Use of the weathering steels has been limited by certain deficiencies in their mechanical properties. Thus, even at their relatively low strength levels (50,000 psi to 60,000 psi yield strengths), they possess limited formability and, in general, poor toughness. Except for a significantly more expensive class of alloy steels, e.g., quenched and tempered steels and so-called Ni-Cu-Cb steels, yield strengths in excess of 60,000 psi are not available in the weathering grades. Further, these more expensive alloy grades only have atmospheric corrosion resistance up to five times that of' carbon steel.

An object of the present invention is to provide lowalloy weathering steels having 80,000 psi minimum yield strengths developed directly off a hot-strip mill. Another object of the invention is to provide such steels having five to eight times the atmospheric corrosion resistance of carbon steels and a desirable balance of mechanical properties, including formability, weldability, fatigue resistance, and toughness. Still another object of the invention is to provide such steels at low cost.

We accomplish the foregoing objects of our invention by providing steels of unique chemistry processed in a critical manner. The steels of our invention have the following chemistry: carbon, .04 to .10 percent; manganese, 0.40 to 1.8. percent; sulfur, 0.03 percent maximum; aluminum, .015 percent minimum; chromium, .90 to 1.20 percent; copper, .30 to .50 percent; silicon, .50 to 1.20 percent; phosphorus, .10 to .15 percent; columbium, .015 to .040 percent; and zirconium, .04 to .12 percent, or rare earths in amounts such that the weight ratio of total rare earths to sulfur is at least 2.8 to 1. The steels are produced according to conventional steel-making techniques with the various alloy additions typically being made to the steels is in a ladle after they have been fully killed. In processing the steel to strip or plate, it is hot-rolled so as to have a finishing temperature between its A temperature and 1,700 F, cooled at a rate between 20 F/sec. and 45 F/sec. and coiled or piled at a temperature of 1,100 F i 100 F. As set out in detail below, the composition of the steels and the method by which they are hotrolled are critical to the formation of the desired balance of properties of corrosion resistance, strength, formability, toughness and weldability.

The steels of our invention derive their corrosion resistance from the elements phosphorous, copper, chromium and silcon. Nickel also can be used if desired. We have found that of these elements phosphorus is the most effective in enhancing corrosion resistance in all environments, i.e., rural, marine and industrial. Following phosphorus in order of effectiveness in both the rural and marine atmospheres are copper, silicon or nickel, and chromium. In the marine atmosphere, however, silicon is more effective than nickel. In an industrial atmosphere, copper, nickel, silicon and chromium follow phosphorus in order of corrosion resistance effectiveness.

While phosphorus is very effective with respect to imparting corrosion resistance and also inexpensive, phosphorus additions are limited by metallurgical considerations. Thus, high phosphorus levels cause a small decrease in ductility and can also cause weld embrittlement due to the strong effect of phosphorus on steel hardenability, especially at carbon contents above about .10 percent. Accordingly, the steels of the inven tion include .10 to .15 percent, preferably .13 percent, phosphorus, and, to counteract any detrimental effects of the phosphorus, the carbon level preferably is maintained at or below .08 percent.

Nickel which is only moderately effective in promoting corrosion resistance in rural, marine and industrial atmospheres is expensive, and, in addition, nickel does not result in any significant improvement in the other properties of ferrite-pearlite' steels. For these reasons, we prefer not to include nickel in the steels of the invention, although, if desired, it can be added in amounts up to about 1 percent.

The steels of the invention include .50 to 1.20 percent, preferably .60 percent, silicon. We have found silicon to be as effective in preventing corrosion resistance as nickel but much less expensive. Silicon also is a solid solution strengthener, and, in amounts recommended for the steels of the invention, has no detrimental effects on the other desired mechanical properties of the steels. But, because of the high silicon content, the steels are aluminum-killed before silicon additions are made to prevent the formation of massive silicate inclusions.

.30 to .50 percent, preferably .35 percent, copper and .90 to 1.20 percent, preferably 1.00 percent, chromium are considered by us to be necessary to impart the desired degree of corrosion resistance to the steels. Copper also results in some precipitation strengthening of the steels, but included in amounts greater than about 0.60 percent can cause hot-shortness. Nickel additions to high-copper steels prevent hot shortness, but where the steels are free of nickel, as in our preferred chemistry, copper contents should be kept at or below 0.50 percent.

We have determined that to secure maximum atmospheric corrosion resistance at a reasonable cost, the phosphorus, silicon, copper and chromium contents of the steels, besides being within the ranges set out above, must be such as to satisfy the following relationship:

21.50(% P) 4.50(% Cu) 1.20(% Cr) 2.20(% Si) s 7.1.

tion strengthening is provided by the corrosion resistant elements discussed above and by manganese additions in the range of 0.40 to 1.80 percent. Grain refinement is achieved by a controlled hot-rolling practice, as discussed below, using columbium as the grain refining agent.

Columbium also acts to strengthen the steels by the precipitation of CbC during hot rolling. The role of columbium as both a grain refining and precipitation strengthening agent in conjunction with carbon is shown in Table l. The heats of Table I were processed to simulate a finishing temperature of 1,650 F, a coiling temperature of 1 100 F and a cooling rate between finishing and coiling of between 30 F/sec. and 45 inclusion shape-control agent, certain inclusions occuring in the steels become elongated during hot rolling and aligned parallel to the rolling direction to adversely affect the the formability and transverse toughness of the steels.

We have found that when using rare earths, a minimum weight ratio of total rare earths to sulfur of 2.8 to 1 is required to establish the desired degree of formability in the steel. Also, when using rare earths, the sulfur content of the steel is preferably maintained below 0.015 percent although the sulfur content can be as high as .030 percent. Zirconium can also be used as the inclusion shape-control agent. The amount of zirconium required depends on the nitrogen content of the F/sec. steel. The steels of our invention typically contain TABLE 1 HEAT NO. 4682 4684 2235 Chemical Analysis (weight percent) Carbon .08 .07 .03 Manganese .77 .88 .62 Sulfur .014 .013 .011 Aluminum .040 .041 .040 Chromium 1.16 1.14 .93 Copper .44 .44 .40 Silicon .53 .48 .46 Phosphorus .12 .13 .12 Cerium (other rare earths not analyzed) .019 Zirconium .089 .ll Columbium .026 .002 .022 Yield Strength (psi) 84,900 52,700 68,800 Tensile Strength (psi) 101,500 74,800 79,800 Percent Total Elongation in 2" 21.3 34.5 30.5 5071 FATT (F) Longitudinal 6 +40 l5 Transverse 0 +40 N.A. Shelf Energy (ft.-lb.)*

Longitudinal 32 64 Transverse 23 18 NA.

FAT'I'. Frnclure Appearance Transition Temperature Impact data hlulnctl on 'u-inch V-notcli ('hurp) specimens We have found with respect to the effect of columbium on yield strength that yield strengths in excess of 80,000 psi can consistently be obtained at columbium contents between about .02 and .03 percent. However, columbium levels as low as about .015 percent can be employed provided higher cooling rates are used. Conversely, we do not consider it necessary to add columbium in excess of .04 percent.

The steels of the invention exhibit superior formability and transverse toughness. Thus, typically, subjected to the ASTM E290 bend test (bend axis perpendicular to the direction of rolling with machined-edge specimens), the steels of our invention can be bent without cracking about an inside radius as small as that equal to the thickness of the steel. Of course, in most commercial fabricating operation, the bend axis is often parallel to the direction of rolling, and the edges are not machined, but usually sheared. Under these conditions, the minimum inside bend radius of our steels is as small as two times the steel thickness (for thicknesses .250inch and less). These properties are established through the incorporation into the steels of an inclusion shape-control agent comprising zirconium or rare earths. Rare earths which can be employed are cerium, lanthanum, praseodymium, neodymium, yttrium, scandium and mischmetal (a mixture of rare earths). The inclusion shape-control agents cause the sulfide inclusions in the steels to retain a spherical form, resulting in a significant improvement in the formability and transverse toughness of the steels. In the absence of an about .006 percent nitrogen, and we have found that zirconium contents of .04 to .12 percent, preferably .08 percent, are required to provide the desired degree of formability.

' As previously noted, the steels of the invention are subject to a controlled hot-rolling practice to bring about the desired mechanical properties directly off the hot mill. In this regard, we have determined that the finishing temperature, the coiling temperature and the rate of cooling between finishing and coiling are all critical to the establishment of the desired strength and toughness in the steels.

To possess yield strengths in excess of 80,000 psi, the steel, of the invention must be hot-rolled finished below about 1,700 F. This is shown in Table II which lists the processing practice and physical properties for steel samples having the following chemistry: carbon, .08 percent; manganese, .77 percent; sulfur, .014 percent; aluminum, .040 percent; chromium, 1.16 percent; copper, .44 percent; silicon, .53 percent; phosphorus, .12 percent; columbium, .026 percent; cerium, .019 percent (other rare earths not analyzed).

TABLE 11 CASE 1 2 Processing Practice TABLE ll-Continued CASE 1 2 Yield Strength (psi) 84,900 74,000 Tensile Sgrength (psi) 101,500 88,800 Percent Total Elongation in 2 21.3 32.0 50% FA'l'l' (F) Longitudinal +6 +14 Transverse 0 +30 Shelf Energy, fL-lb.

Longitudinal 35 46 Transverse 23 32 As can be seen, finishing temperatures above 1,700 E, in addition to causing the yield strength to fall below 80,000 psi, impair somewhat the fracture appearance transition temperature (FATT). To minimize production liabilities as a result of low line speeds required when using low finishing temperatures, the finishing temperature is usually maintained above about 1,550

We have determined that the steels of the invention must be cooled between the end of hot rolling and coiling at a rate in excess of about 20 F/sec. to consistently We prefer to maintain the cooling rates at between about 20 F/sec. and 45 F/sec. because at these rates yield strengths of 80,000 psi are assured. It is, of course, possible to employ higher cooling rates, particulary when the columbium content is as low as about .015 percent or if yield strengths of 90,000 psi or greater are desired; but uniform elongation decreases with increasing cooling rates and existing commerical requirements do not demand yield strengths in excess of about 80,000 psi.

The yield strengths of the steels 0f the invention fall off above and below a coiling temperature of about 1,100 F in a manner such that unless the coiling temperature is maintained at 1,100 F i 100 F, yield strengths lower than 80,000 psi will result. This is shown in Table IV which lists the processing practice and physical properties for steel samples having the following chemistry: carbon, .08 percent; manganese, .77 percent; sulfur, .014 percent; aluminum, .04 percent; chromium, 1.16 percent; copper, .44 percent; silicon, ,5 3 percent; phosphorus, 12 percent; columbium, .026 percent; cerium, .019 percent (other rare earths not analyzed).

TABLE lV CASE 1 2 3 Processing Practice Coiling Temperature, "F 900 1100 1300 Finishing Temperature, "F 1650 1650 1650 Cooling Rate, F/sec. 30 to 45 30 to 45 30 to 45 Yield Strength (psi) 59,400 84,900 62,000 Tensile Strength (psi) 96.700 101,500 79,500 Percent Total Elongation in 2" 26.8 21.3 32.0 507: FATT (F) Longitudinal 2 +6 +40 Transverse 0 0 +40 Shell Energy, ft. 1b.

Longitudinal 47 35 26 Transverse 34 23 25 develop yield strengths in excess of 80,000 psi. This is shown in TABLE 111.

TABLE Ill HEAT NO. 4682 4609 Chemical Analysis (weight percent) Carbon .08 .08 Manganese .77 .65 10 Sulfur .014 .016

Aluminum .040 .040 Chromium 1.16 .97 10 Copper .44 .43 Silicon .53 .42 Phosphorus .12 .12 Columbium .026 023 Cerium (other rare earths not analyzed) .019 Zirconium .099 Processing Practice Cooling Rate, "Est-1:. to 45 1O Finishing Temperature. "F 1650 1650 Coiling Temperature, F 1 100 1100 Yield Strength (psi) 84,900 62,700 Tensile Strength (psi) 101,500 82,600 Percent Total Elongation in 2 21.3 30.5 50% FATT ('F) 10 Longitudinal +6 +45 Transverse 0 N.A. Shelf Energy, ft.-lb.

Longitudinal 47 Transverse 23 NA.

Cooling rates below about 20 F/sec. result in low strengths and increased transition temperatures.

We have found the steels of the invention to have good welding properties. The joint efficiencies approach percent with no excessive hardness increase or reduction in toughness in the heat affected zone. Welded samples of the steels with sheared edges can be bent over an inside radius equal to 1.5 times plate thickness with the bend axis parallel to the rolling direction without the occurrence of cracking. In addition, toughness of the heat affected zone is comparable to the base metal.

We claim:

1. A process for producing low-alloy steels characterized by having 80,000 psi mimimum yield strengths, five to eight times the atmospheric corrosion resistance of carbon steels, and improved formability and toughness comprising providing steels having the following chemistry: carbon, .04 to .10 percent; manganese, .40 to 1.80 percent; sulfur, .03 percent maximum; chromium, .90 to 1.20 percent; copper, .30 to .50 percent; silicon, .50 to 1.20 percent; phosphorus, .10 to .15 percent; columbium, .015 to .040 percent; sufficient aluminum to kill the steel; an inclusion shape-control agent selected from the group consisting of .04 to .12 percent zirconium and rare earths in amounts such that the weight ratio of total rare earths to sulfur is at least 2.8 to 1; and balance essentially iron, the phosphorus, copper, chromium and silicon contents being such as to satisfy the relationship:

6.2 s 2l.50(%P) 4.50(%Cu) l.20(%cr) 2.20(%Si) s 7.1,

hot-roll finishing the steels at a temperature between their A temperature and l,700 F, cooling the steels at a rate above 20 F/sec. and coiling or piling the steels at a temperature of 1,100 F i l F.

2. The process of claim 1 wherein the steels are cooled at a rate of between F/sec. and 45 F/sec.

is at least 2.8 to 1. 

2. The process of claim 1 wherein the steels are cooled at a rate of between 20* F/sec. and 45* F/sec.
 3. The process of claim 2 wherein the inclusion shape-control agent comprises .04 to .12 percent zirconium.
 4. The process of claim 2 wherein the inclusion shape-control agent comprises rare earths in amounts such that the weight ratio of total rare earths to sulfur is at least 2.8 to
 1. 